A thread cutting device is disclosed German Patent Document DE 40 15 707 C2 which has a yarn cutting blade disposed between the core-sleeve receiving portion of the spindle and a yarn-collecting region, e.g. a knurled cylindrical portion, below the spindle. This blade has a downwardly extending edge radially spaced from the spindle and on a frustoconical portion of the blade.
In practice, after the last turns of the yarn are wound on the bobbin or spool formed on the core sleeve by control of the ring rail so that these turns will not loosen, and prior to doffing of the bobbin or spool, the ring rail of the spinning or twisting machine is controlled to wind two or more turns on the yarn collecting cylindrical portion of the spindle below the blade, so that the yarn will be located against the blade between the last turns on the spool or bobbin and the turns wound on the yarn-collecting region.
When the bobbin or spool is then removed from the spindle, e.g. by an automatic bobbin or spool removal or doffer, the yarn strand lying against the blade is severed by a cutting edge thereof.
The fabrication of such a blade is costly and time consuming. The frustoconical region has an open angle of 45.degree. (complement to the angle of the frustocone or the angle included between a generatrix of the outer surface of the cone and a line parallel to the axis of the spindle and of the blade, i.e. a vertical in most cases). The cutting edge can then be ground at 30.degree. to leave an angle of 15.degree.. Because of this very special configuration, the blades must be mounted individually and equally along both the inner and outer sides of the frustoconical apron. Frequently, the blades do not function fully satisfactorily when individually ground in this manner.